Radiation detectors for computed tomography scanners generally comprises rows or matrices of so-called pixels arranged adjacent to one another, that is to say of individual radiation detector modules which directly or indirectly convert incident (X-ray) radiation into electrical signals which can subsequently be used to derive projection images therefrom, which images can be processed further for reconstructing an image of an examination object to be imaged.
In the case of indirect conversion of the incent radiation, so-called scintillation elements are used, which first of all convert the radiation into light (usually in the visible wavelength band). The scintillation elements are separated from one another by so-called septa which have light-reflecting materials inserted into them in order to prevent light pulses generated in the individual elements from passing into adjacent pixels. Likewise, the radiation inlet surface toward the interior of the scintillation elements is designed to reflect. A light detector arrangement, for example in the form of a photodiode, then converts the light into electrical signals.
Until now, the light detector arrangements were attached below the scintillation elements, i.e. the light inlet surfaces thereof used to be parallel to the radiation inlet surface of the scintillation elements. The electronic system used for further signal processing, preferably converter elements for the electrical signals, was then attached to the side of the light detector arrangements opposite to the radiation inlet surface.
For quite some time now, the tendency has been to reduce the size of the pixels in the z- and φ-directions, that is to say in the directions which approximately form a plane which is basically aligned perpendicularly with respect to the main direction of the radiation to be detected. In the following text, the main direction of the radiation to be detected is considered to be the direction of propagation of the radiation in which the substantial portion of the radiation to be detected is incident on the radiation detector and which can for example be defined by a collimator or the like arranged upstream of the detector.
In the process, it is usually ensured that the radiation to be detected is basically incident on the radiation inlet surface in a perpendicular fashion, that is to say that the main direction is perpendicular with respect to the radiation inlet surface. Basically perpendicular should in this case respectively be understood to mean that the respective directions are perpendicular with respect to one another within certain tolerances. The reduction of pixel size can achieve both higher time and spatial resolutions in radiation detectors. However, in the arrangement of radiation detector modules with scintillation elements and light detector arrangements attached to the underside described above, this also automatically reduces the size of the detection surface of the light detector arrangements and leads to the undesired effect that increasing the miniaturization reduces the number of light quanta generated and the light collection becomes less and less efficient. In the end, the strength of the measurement signals is reduced and so interference effects such as more noticeable noise become more emphasized.
WO 2006/114715 A2 and WO 2006/114716 A2 point toward a solution to this problem. Therein, the light detector arrangements are in each case not on the underside of the scintillation elements, but are on the side between the scintillation elements. Since the pixel heights can, in contrast to the extents parallel to the detector surface, only be varied slightly, a sufficient size of the light detector arrangements is maintained for the light collection efficiency when there is such a lateral readout of the light signals, even when the pixels are reduced in size.
However, a further problem during the miniaturization of the scintillator pixels consists of the fact that the number of pixels per unit area or per detector length unit attained thereby, that is to say the so-called pixel density, makes the signal processing more complicated. The technical limits of producible printed circuit board densities are already being reached these days. If anything, this problem is further increased in the case of a lateral arrangement of the light detector arrangements, because now the electronic system which used to have space on the underside of the light detector arrangements now no longer has sufficient space.